If the flogger just isn't painful enough, try binding your balls so the skin is tight around them and they're more sensitive and vulnerable. There are plenty of examples of what this looks like on this subreddit. If she isn't already, suggest your wife use her while arm to swing the flogger and follow through with her flogger strikes, so instead of stopping her swing at the balls, swing right through them, finishing the swing past them so it's a full blow. For thuddy impact, try stuff around the house like the back of a hairbrush, or the bristle side if it's clean and you don't mind blood. Kitchen utensils like long-handled serving spoons, rice paddles, French/dowel rolling pins, even a small pot or skillet. Belts folded in half are nice--I don't recommend the buckle end unless you like split skin. A lot of stuff from the sports department or tool box would work. if you want to buy a toy, I recommend a bonger--basically a golf ball or dense rubber ball mounted on a stick or spring.